Heretofore various sorts of communication systems have been investigated and developed. Among them the spread spectrum communication (hereinbelow abbreviated to SSC) system is well known.
By this SSC system, on the transmitter side, a signal such as data, sound, etc. having a wide band to be transmitted by using a pseudo-noise code (PN code) and, on the receiver side, this wide band signal is spread inversely into the original narrow band by means of a correlator to reproduce the signal. Recently attention is paid to this communication system, because it has always a very high reliability from the point of view that it is strong against external interference and noise, that it has a high secrecy, etc.
At present, for the wireless SSC, a correlator, which is thought to be the most simple and convenient and to have a high reliability, is a device using surface acoustic wave (hereinbelow abbreviated to SAW).
In the SAW correlator there are, in general, those of correlator type (tapped delay line type) and those of convolver type. Here, although those of correlator type have a simple construction and generally a high efficiency, the temperature coefficient of the substrate has remarkable influences thereon. On the other hand, although those of convolver type are hardly influenced by variations in the temperature, they have, in general, a low efficiency. In addition, concerning the PN code described above, the code is fixed for those of correlator type, while it can be freely changed for those of convolver type.
Consequently correlators of convolver type are more easily used, provided that the efficiency is at a practically usable level.
On the other hand, as the PN code used in the SSC system, heretofore binary sequences such as an M sequence, a GOLD sequence, etc. have been principally used owing to the simplicity of the code generation. However, since the cross-correlation value of these binary sequences is not always small, in the spread spectrum multiple access (hereinbelow abbreviated to SSMA) communication they cause often cross-talk. As a PN code for the purpose of solving such a problem, recently a square phase sequence (minimum cross-correlation multiple phase orthogonal sequence) as described in JP-A-63-73730 has been proposed.
The square phase sequence is a complex number sequence having a period N (N being a code length), by which the self correlation function is zero except for the shifts, which are integer times as long as the code length N and further the absolute value of cross-correlation functions between different sequences having a same code length is 1/.sqroot.N, when it is nomalized, taking the 0-shift component of the self correlation function as 1. That is, it realizes the mathematical lower limit of the peak value of the absolute value of the cross-correlation function between orthogonal sequences.
Although the square phase sequence is a PN code suitable for the SSMA communication, heretofore no spread spectrum modulating device using the square phase sequence is known.
Further, as a correlator for the square phase sequence, that described similarly in JP-A-63-73730 is known. However no case where an SAW convolver is used as a correlator is studied.